One obvious problem is lack of space. This does vary
considerably from airline to airline, and from airplane
model to airplane model. If you travel a lot, you might
want to sample different airlines and airplanes to determine
your preferences. If you are very tall, you may prefer
an aisle seat. This will hopefully give you a chance
to stretch your legs into the aisle from time to time.
Even better is to get a "bulkhead seat": there is usually
more legroom in those positions. Be advised, however,
that those seats go quickly; it pays to book ahead!

Oddly, something that helps you feel less cramped is
to travel with a friend instead of alone. The better
you are acquainted with someone, the harder it is to
have your "personal space" violated.

Air Sickness
If you get motion sick easily, you may be more comfortable
if you take some sort of motion sickness drug before
you fly. Note that you must take the drugs before you
get sick; there is unfortunately nothing that I know
of that will relieve motion sickness once it has started.

My personal drug of choice is Drammamine? (TM), a readily-obtained
over-the-counter medication. Unlike Bonine? (TM), another
over-the-counter drug, it tends to make one drowsy.
I feel that this is a benefit on a long plane ride!
Another motion sickness treatment (usually used for
boating) is scopalamine patches. These patches go behind
the ear, and usually have no noticeable side effects.
I believe that in the USA, a prescription is required.
Furthermore, there can be unpleasant side effects, such
as dry mouth, blurry vision, and one other one that
I forget. (Oh yeah - loss of memory.)

Motion sickness is caused in part by a discrepancy between
what the eye sees and what the inner ear feels. (This
is why the driver of a car gets sick much less frequently
than a passenger: the driver is continuously watching
the road, getting a good idea of what is coming next.)
It may help to close your eyes the moment you start
to consider thinking about getting airsick.

If you do come down with discomfort, one relatively
easy but embarrassing way to feel better is to empty
your stomach. "Barf bags" are usually located in the
pouch on the back of the seat in front of you. Frequently,
using them reduces and/or eliminates the discomfort.

Air Quality

The air that you breathe inside an airline cabin isn't
nearly as good as what you will find most other places.
The cabin is pressurized, since the air is so thin at
altitude. However, they don't pressurize it to sea level;
it is substantially weaker. (This is part of why air
travel can be so draining; you can end up with mild
altitude sickness!)

Furthermore, the air is very very dry. This can lead
to dehydration, which can also make you feel lousy.
I strongly advise bringing a litre or two of water with
you, and guzzling that non-stop throughout the trip.

Finally, the air is filled with the exhaust products
of your fellow travelers. One of these can be cigarette
smoke; while smoking has been banned on domestic US
flights, such rules are not followed around the globe.
Although it might horrify some Americans, not all countries
even mandate separate smoking sections!

Another exhaust product is germs. Because airplane tickets
are so expensive, and because such effort is involved
in making such a trip, people will fly sick. If your
immune system is compromised in any way (HIV, chemotherapy,
immunoglobulin deficiency), you may well want to wrap
your face in a scarf or wear a surgical mask.

Temperature

Because of the altitude, airplanes can also be quite
cold (especially the floor). I always take a jacket
with me on the plane and take one of the blankets that
the airline provides. Wool socks are not a bad idea
either. On the larger planes, there is usually a little
fan that blows on you. The airflow can be adjusted by
twisting the unit.

Noise

Planes are also very noisy. You might not think this
is a big deal, but it wears away at you. Consider bringing
some earplugs with you. Most of the large jets have
sound systems built-in. Many of the newer planes also
have built-in TV systems for movies and infomercials.
Usually, you have to pay a small fee (generally around
five US dollars) to rent headphones if you want sound.
Be advised that the sound quality will NOT rival that
of a compact disk player: the tapes get played over
and over again, and the lack of fidelity can pain a
purist. You can bring portable tape and/or compact disk
players aboard with you, but airlines may restrict their
use (especially on takeoff and landing). There is some
concern that the electromagnetic fields generated by
consumer electronics can interfere with the airplane's
navigational signals. I don't know if these concerns
have been empirically substantiated, but I don't care.
I'd rather be bored than in a plane crash.

Food

Food on airlines is about what you would expect, considering
that all the food must be prepared ahead of time and
served to a large number of people with very different
taste preferences. It is amazing that the food is as
good as it is, but still, it frequently will not please
you. U.S. carriers are pretty good about accommodating
standard dietary preferences, but you have to help them
out. If you are vegetarian or keep kosher, tell your
travel agent when you purchase the ticket, and the airlines
will usually accommodate you.

If you have food allergies, you are probably safest
bringing your own food with you. (In fact, even if you
eat anything, you are probably better off bringing your
own food with you!) Be advised that many countries have
import restrictions on foods; If you bring food, be
sure that you either finish it all on the plane or make
sure that it will clear customs.

Morley Selver suggests never getting on an airplane
hungry. You might think you will get a meal shortly,
but the following could happen: 1) Everyone boards the
aircraft, then they decide they have to fix something.
They are not sure how long it will take, therefore nobody
is allowed off. 2) You take off on a 3 hour flight that
has 2-1/2 hrs of turbulence where the flight crew is
not allowed to serve meals. 3) You do not like the food.
4) There is an electrical problem with the galley and
your half of the plane does not get a meal.